In the optical player a reading beam scans a spiral track of a video record, such as a disc, to derive the program information. Most frequently the information is stored in the disc under the control of a carrier signal that is angle-velocity-modulated (frequency modulated) with luminance information and with a subcarrier signal that is amplitude and phase modulated with chroma information. The sound information of the program is conveyed by one or more audio modulated subcarriers also used in preparing the record disc and the position of these various signals within the frequency spectrum is subject to choice. That choice is customarily made for optimum conditions of recording and playback. One may, for example, transcode the signals so that their frequency assignments are related to, but specifically different from that of the NTSC color signal specifications. As the art has developed, it has become possible to record the program in the NTSC format. This is especially desirable where the character of the playback apparatus is such that the carrier signal modulated by program information may be derived from reading the disc with such precision that it may be utilized directly, obviating the need for further signal processing or transcoding.
In order to be able to use the output signal of the playback apparatus without further signal processing or transcoding, it is necessary that timing errors, attributable for example to eccentricities of the disc, be compensated and maintained within an allowable tolerance range. Timing errors, when experienced, adversely affect both the luminance and chroma information but there is considerable more latitude with respect to the former than the latter. In point of fact, deficiencies in chromaticity of the reproduced image are manifest if the phase error of the chroma subcarrier is in excess of approximately 10 nanoseconds. Compensation of timing errors is accomplished by a correction system controlled to displace the reading beam tangentially of the reading point in the sense and amount required for compensation. Manifestly, the requirements of the timing correction system are exacting if the compensation is to be accomplished with the precision necessary to maintain phase coherence of the chroma subcarrier signal.
Applicant has devised a phase locked servo-type of timing correction system that satisfies the precision requirements and yields an output signal in which the luminance information is corrected for timing errors and in which the chroma information has adequate phase coherence for use by a color receiver. Such a servo system makes it possible to play back a video disc with little, if any, difficulty if the recording is uninterrupted from a starting point to the finish. Some difficulty will be experienced, however, in intervals of stop frame reproduction which, from a practical standpoint, is a unique and especially attractive attribute of the optical video disc and playback arrangement. This difficulty is inherent in the NTSC signal specifications which prescribe that the chroma subcarrier signal correspond to an odd multiple of one-half the line scanning frequency. Because of that frequency relation, the phase of the chroma subcarrier signal changes 180.degree. from line-to-line as well as from frame-to-frame. As a consequence, during stop frame operation when the reading beam returns from the end of an image frame to the beginning of that frame, the phase of the chroma signal monitored by the phase locked servo system undergoes an abrupt excursion of 180.degree.. This renders the servo unstable and the operation of the playback device becomes imperfect.
Accordingly, it is a principal object of the invention to accommodate video playback systems to those characteristics of the NTSC format that otherwise may impair the performance of the playback arrangement, especially during intervals of stop frame.
It is a specific object of the invention to afford such accommodation where the playback features a phase locked servo-type of timing correction system.